In an information recording medium, such as a CD-ROM (Compact Disc-Read Only Memory), a CD-R (Compact Disc-Recordable), a DVD-ROM, a DVD-R, a DVD-RW, and a DVD+R, for example, as described in patent documents 1 and 2 or the like, there is also developed an information recording medium, such as a multilayer type or dual layer type optical disc, in which a plurality of recording layers are laminated or pasted on the same substrate. Then, on an information recording apparatus, such as a DVD recorder, for performing the recording with respect to the dual layer type, i.e., two-layer type, optical disc, laser light for recording is focused or condensed on a recording layer located on the front (i.e. on the closer side to an optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L0 layer”, as occasion demands) to thereby record information into the L0 layer in an irreversible change recording method by heat or a rewritable method. Moreover, the laser light is focused or condensed on a recording layer located on the rear of the L0 layer (i.e. on the further side from the optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L1 layer”, as occasion demands), through the L0 layer or the like, to thereby record information into the L1 layer.
When information is recorded into the L1 layer, which constitutes such a two-layer type optical disc, the optimum recording power of the laser light with which the L1 layer is irradiated through the recorded L0 layer, as shown in FIG. 11(a), is 44.5 (mW: milliwatt) at which a jitter value is minimal on a parabolic curve in a thin line (with white triangles) in FIG. 11(c), for example. On the other hand, the optimum recording power of the laser light with which the L1 layer is irradiated through the unrecorded L0 layer which has a different light transmittance from that of the recorded L0 layer, as shown in FIG. 11(b), is 46 (mW: milliwatt) at which a jitter value is minimal on a parabolic curve in a thick line (with black triangles) in FIG. 11(c), for example. Thus, there is a need to consider whether or not the L0 layer is recorded, in the case of the recording in the L1 layer. With respect to this, there is devised or invented a recording method in which a so-called recording order is satisfied, which is that the laser light for recording which has penetrated or transmitted the L0 layer in a recorded state is to be irradiated, for example.
However, in producing such a two-layer type information recording medium, the L0 layer and the L1 layer are formed by different stampas, and are laminated or pasted. Thus, there is a possibility to cause an eccentricity due to an error in lamination or pasting, in the L0 layer and the L1 layer. Alternatively, since the L0 layer and the L1 layer are formed by different stampas, there likely arises deviation in a track pitch in each recording layer, or there likely arises deviation, a so-called dimensional error, in an absolute radial position with respect to a reference address in each recording layer. These cause a shift in the radial position of a recording area in the L1 layer which is associated with a recording area in the L0 layer by address information, such as a pre-format address, for example, and thus there arises a possibility that the above-mentioned recording order is not necessarily satisfied. More specifically, it is assumed that the recording is performed with a recording power which is optimized in the recording after the penetration of the recorded L0 layer. Also it is assumed that when the information is recorded into the L1 layer, as shown in a left part of FIG. 12, the recording power is set so as to perform appropriate recording in the L0 area in the recorded state. In this case, in the area where the laser light for recording which has penetrated the L0 layer in the recorded state is irradiated, the amplitude of a reproduction signal is large, and good signal quality is obtained. In other words, an asymmetry value obtained from this single is appropriate. On the other hand, as shown in a right part of FIG. 12, if the recording power is set so as to perform the appropriate recording in the L0 area in the recorded state, in the area where the laser light for recording which has penetrated the L0 layer in the unrecorded state is irradiated, the amplitude of a reproduction signal is small, and good signal quality is not obtained. In other words, the asymmetry value obtained from this signal is far from the appropriate value, like having a low asymmetry value. On the other hand, as shown in a middle part of FIG. 12, in the area where the laser light for recording which has penetrated the L0 layer in which the recorded area and the unrecorded area are mixed is irradiated, the amplitude of a reproduction signal varies depending on the extent of an eccentric amount (or radial run-out); namely, depending on how disproportionately the L0 area in the recorded state or the L0 area in the unrecorded state is located from the center of the laser light, in the circle. In other words, the asymmetry value obtained from this signal transits from one to the other out of the high level and the low level.
In order to eliminate the deviation of the optimum recording power due to the relative shift, such as the above-mentioned pasting error (or bonding error) and dimensional error, if a recording apparatus detects the recording state of the recording area in the L0 layer which is associated with the recording area in the L1 layer, a recording control process becomes complicated because it is necessary to accurately recognize the above-mentioned relative shift. On the other hand, if the information is recorded in disregard of the deviation of the optimum recording power due to the relative shift, the control becomes complicated; for example, a process parameter for obtaining a binary signal is to be dynamically changed, on a reproducing apparatus for reproducing the recorded information, which possibly causes troubles in the reproduction process.
Thus, the inventors of the present invention have proposed a method of defying in advance a relationship between an address and a physical radial position so as not to cause troubles in the reproduction quality of the recorded information even if the recording is performed with the recording power being constant in the normal case between the L0 layer and the L1 layer. Specifically, an information recording medium is prepared such that a radial position in the address system of the L1 layer corresponding to that of the L0 layer is located on the inner side than that of the L0 layer.
Moreover, the inventors of the present invention have also proposed a method of satisfying the recording order by shifting the radial position of an edge indicating the innermost or outermost circumference of a recording area formed in the L1 layer, to the outer or inner circumferential side than the radial position of an edge of a recording area formed in the L0 layer, by a margin amount determined to reduce an influence of the above-mentioned relative shift.
Patent document 1: Japanese Patent Application Laid Open NO. 2000-311346
Patent document 2: Japanese Patent Application Laid Open NO. 2001-23237